Overview

Shoulder injuries are common in both young, athletic people and the aging population. In each of these age groups, there are numerous causes of shoulder pain. Two of the most common problems occur in the narrow space between the bones of the shoulder. Irritation in this area may lead to a pinching condition called impingement syndrome, or damage to the tendons known as a rotator cuff tear. These two problems can exist separately or together. It is likely that rotator cuff tears are the result of impingement syndrome and age related changes within the rotator cuff tendons.

What does the inside of the shoulder look like?

The shoulder is the most mobile joint in the human body, with a complex arrangement of structures working together to provide the movement necessary for daily life. Unfortunately, this great mobility comes at the expense of stability. Four bones and a network of soft tissue structures (ligaments, tendons, and muscles), work together to produce shoulder movement. They interact to keep the joint in place while it moves through extreme ranges of motion. Each of these structures makes an important contribution to shoulder movement and stability. Certain work or sports activities can put great demands upon the shoulder, and injury can occur when the limits of movement are exceeded and/or the individual structures are overloaded.

Bones and Joints

The four bones of the shoulder:

  • The humerus is the upper arm bone. This is the "ball" of the shoulder's "ball and socket" joint.

  • The scapula is the flat, triangular bone commonly called the shoulder blade. Prominent areas of the scapula serve as attachment points for many muscles and ligaments.
         - The glenoid is the shallow "socket" on the side of the scapula that receives the 'ball' of the humerus. Together they form the "ball and socket" arrangement of the shoulder.
         - The scapular spine is a horizontal ridge along the back of the scapula that divides the scapula into upper and lower regions.
         - The acromion is the end of the scapular spine. It projects up to form the top of the shoulder.
         - The coracoid process is a projection towards the front of the scapula and is an attachment site for several muscles and ligaments.

  • The clavicle is the collarbone. Although it appears to be straight, it actually forms an S-shape when seen from above.

  • The thorax, or rib cage, is an anchor for several muscles and ligaments. Although the ribs do not physically attach to the scapula, the thorax stabilizes and maintains proper positioning of the scapula so that the arm can function to its fullest capacity.

Together these four bones form four junctions, or joints:
  • The glenohumeral joint is the main joint of the shoulder. Here, the glenoid on the scapula and the head of the humerus come together. The fairly flat socket of the glenoid surrounds only 20% - 30% of the humeral head. Because of its poor fit, this joint relies heavily on the surrounding soft tissue for support. The labrum, a ring of fibrocartilage tissue, attaches to the glenoid and deepens the socket to encircle more of the humerus.
  • The acromioclavicular joint, or AC joint, is the bony point on the top of the shoulder. It stabilizes the scapula to the chest, by connecting the acromion on the scapula to the clavicle, or "collarbone". A thick disk of fibrocartilage acts as a shock absorber between the two bones. The surrounding capsule and ligaments give this joint great stability.
  • The sternoclavicular joint, or SC joint, connects the other end of the clavicle to the sternum, or "breastbone". Like the AC joint, this joint contains a fibrocartilage disk that helps the bones achieve a better fit. It also gets excellent support from its joint capsule and surrounding ligaments.
  • The scapulothoracic articulation is the area where the scapula, embedded in muscle, glides over the thoracic rib cage. The surrounding muscles and ligaments keep the scapula properly positioned so that the arm can move correctly.
  • Cartilage

    There are two types of cartilage in the shoulder:

  • Articular cartilage is the shiny white coating that covers the end of the humeral head and lines the inside surface of the glenoid. It has two purposes:
         - To provide a smooth, slick surface for easy movement
         - To be a shock absorber and protect the underlying bone

  • Fibrocartilage is the thick tissue that forms the disks of the AC and SC joints and the labrum, the ring that deepens the glenoid. Fibrocartilage has three roles:
         - To act as a cushion in shock absorption
         - To help stabilize the joint by improving the fit of the bones
         - To act as a spacer and improve contact between the articular cartilage surfaces

Ligaments

The shoulder relies heavily on ligaments for support. Ligaments attach bone to bone and provide the "static" stability in a joint. Ligaments will alternately become tight and loose with normal motion. They keep the joint within the normal limits of movement.

  • The glenohumeral ligaments attach in layers from the glenoid labrum to form the joint capsule around the head of the humerus.

  • The coracoacromial arch is the group of ligaments that spans the bony projections of the coracoid process and the acromion.

Muscles and Tendons

Many muscles and tendons work together in the shoulder to provide the wide range of movements necessary for daily living and sport. These muscles and tendons provide the "dynamic" stability of the shoulder.

There are four muscle groups in the shoulder:

  • The rotator cuff muscles are the subscapularis, the supraspinatus, the infraspinatus, and the teres minor. They are the primary stabilizers that hold the "ball" of the humerus to the glenoid "socket". The socket is too shallow to offer much security for the humerus. These four muscles form a "cuff" around the humeral head, securing it firmly in the socket. As its name implies, this group of muscles also rotates the arm. The rotator cuff protects the glenohumeral joint from dislocation, allowing the large muscles that control the shoulder to power the arm with great mobility.

  • The biceps tendon complex also helps keep the humeral head in the glenoid and helps raise the arm.

  • The scapulothoracic muscles attach the scapula to the thorax. Their main function is to stabilize the scapula to allow for proper shoulder motion.

  • The external muscles of the shoulder are the large, powerful muscles important to the overall function of the shoulder. This group includes the deltoid muscle, which covers the rotator cuff muscles.

Bursae

A bursa is a pillow-like sac filled with a small amount of fluid. Bursae (plural) reduce friction and allow smooth gliding between two firm structures, like bone and tendon or bone and muscle. There are over 50 bursae in the human body; the largest is the subacromial bursa (under the acromion) in the shoulder. The subacromial bursa and the subdeltoid bursa (under the deltoid muscle) are often considered as one structure. This bursa separates the rotator cuff and the deltoid muscle, from the acromion.

What is impingement syndrome?

Shoulder impingement syndrome occurs when the tendons of the rotator cuff and the subacromial bursa are pinched in the narrow space beneath the acromion. This causes the tendons and bursa to become inflamed and swollen. This pinching is worse when the arm is raised away from the side of the body. Impingement may develop over time as a result of a minor injury, or as a result of repetitive motions that lead to inflammation in the bursa.

Particular shapes of the acromion may make certain individuals more susceptible to impingement problems between the acromion and the bursa. With age and the onset of arthritis, the acromion may develop bone spurs that further narrow this space. Impingement caused by bone spurs on the acromion is common in older patients who participate in sports or work activities that require overhead positions. Spurs may also result if one of the ligaments in the coracoacromial arch becomes calcified.

Impingement is classified in three grades:

  • Grade I is marked by inflammation of the bursa and tendons

  • Grade II has progressive thickening and scarring of the bursa

  • Grade III occurs when rotator cuff degeneration and tears are evident

What is a rotator cuff tear?

Continual irritation to the bursa and rotator cuff tendons can lead to deterioration and tearing of the rotator cuff tendons. The tendon of the supraspinatus muscle is the most commonly involved tendon among the rotator cuff muscles. This muscle forms the top of the cuff and lies in the narrow space beneath the acromion. It is subject to the most pinching of all the rotator cuff muscles.

Rotator cuff tears can be the result of a traumatic injury or deterioration over time. Symptoms may be present, but in many cases, the patient experiences no symptoms at all. In young active people, full thickness rotator cuff tears are fairly uncommon. When they do occur, they are usually the result of a high-energy injury to the rotator cuff that is associated with throwing or overhead sporting activities. In older people, rotator cuff tears tend to be the result of wear and tear over time. Several scientific studies have shown that up to 2/3 of the population at age 70 have rotator cuff tears; many of these people had no symptoms.

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